The newest study of the variation of U isotope ratios in water and sediments in the karst aquifer of Krka River in Croatia showed that U isotopes are identifiers of authigenic carbonate and can be used for quantification of terrestrial CO2 sinks related to carbonate weathering.
The concentration and isotopic composition of uranium (δ238U, 234U/238U activity ratio) in combination with traditional isotopes (δ18O, δ13C) were examined as potential tracers of authigenic carbonate formation in a karst aquifer. The U concentration and 234U/238U activity ratios in the tufa-precipitating sections of two connected karst rivers (Krka and Zrmanja, Croatia) decreased downstream in water and in precipitated carbonate due to active selfpurification processes, i.e. adsorption of isotopically lighter U(VI) on mineral particles, sedimentation and coprecipitation with carbonate.The isotopic composition of carbonate in tufa mostly resembled the 234U/238U activity ratio and the δ238U values of dissolved U in water, but was also affected by the presence of detrital carbonate flushed into the river from soil and weathered bedrock. This interpretation was supported by the δ18O and δ13C values of tufa, which were shifted out of equilibrium with river water and dissolved inorganic carbon and in their isotopic signature, which showed the presence of lithic carbonate. Large fluctuations of the δ238U values of water, leachable U (eluted in acetic acid buffered with Na-acetate) and residual U fraction could not be fully explained by available data due to the overlapping U isotopic signatures of leachable (mainly carbonate) and residual fractions of soil, bedrock and tufa. Therefore, a long-term, systematic, seasonal and event-based observation of the isotopic composition of dissolved and suspended particulate U in water is necessary.
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